Larval zebrafish exhibit collective circulation in confined spaces

Collective behavior may be elicited or can spontaneously emerge by a combination of interactions with the physical environment. To investigate the relative contributions of these factors for a millimeter-scale swimming organism, we observed larval zebrafish interacting at varying densities under circular confinement. If left undisturbed, larval zebrafish swim intermittently in a burst and coast manner and are socially independent at this developmental stage, before shoaling behavioral onset. We explore the behavior these larvae as they swim together inside dishes. We report our analysis of a new observation for this well-studied species: in circular confinement and at sufficiently high densities, the larvae collectively circle rapidly alongside the boundary. This is a new physical example of self-organization of mesoscale living active matter driven by not only by boundaries and environment geometry, but also influenced by social interactions.